Field
This disclosure relates to communication from downhole tools to the surface and among different sections of the bottom hole assembly (BHA).
Background Discussion
Directional drilling uses a BHA in the drill string, which typically includes a drill bit, stabilizers, bent subs, drill collars, rotary steerable and/or a turbine motor (mud motor) that is used to turn the drill bit. In addition to the BHA, a set of sensors and instrumentation, known as a measure while drilling system (MWD), is normally required to provide information to the driller that is necessary to guide and safely drill the borehole. Due to the mechanical complexity and the limited space in and around the BHA, the MWD is typically placed some distance from the bit above the motor assembly. A communication link to the surface is typically established by the MWD system using one or more means such as a wireline connection, mud pulse telemetry or electromagnetic wireless transmission. Because of the lag between the bit location and the sensors monitoring the progress of the drilling, the driller at the surface may not be immediately aware that the bit is deviating from the desired direction or that an unsafe condition has occurred. For this reason, drilling equipment providers have worked to provide a means of locating some or all of the sensors and instrumentation in the limited physical space in or below the motor assembly and therefore closer to the drill bit while maintaining the surface telemetry system above the motor assembly. These sensors generate near-bit data that is typically communicated to the MWD section to be transmitted to the surface.